The present invention relates to a process for producing rigid, dimensionally stable, polyurethane foams in which the amount of pressure generated during the foaming process is reduced and to the foams produced by this process.
Rigid polyurethane foams and processes for their production are known. Such foams are typically produced by reacting an isocyanate with an isocyanate-reactive component such as a polyol in the presence of a surfactant, water, blowing agent and a catalyst.
The processing and properties of the foam may be affected by the specific isocyanate, isocyanate-reactive component, blowing agent, catalyst and/or additives used. The catalyst is known to affect the rate of blowing gas generation and gelation.
U.S. Pat. No. 6,432,864, for example, teaches that if a catalyst promotes the isocyanate-water (blowing) reaction to too high a degree, carbon dioxide will be evolved before sufficient reaction of isocyanate with polyol (gelling) has occurred and the resultant foam will collapse. If, however, the catalyst promotes the gelling reaction too strongly, a substantial amount of carbon dioxide will be generated after a substantial degree of the polymerization has already occurred thereby producing a foam with broken or poorly defined cells. The balance sought between promotion of the blowing action and of the gelling action is achieved in this disclosure by using an acid-blocked amine catalyst corresponding to a specified structure.
In U.S. Pat. No. 6,384,097, the blowing reaction was delayed while the gelling reaction was promoted in order to produce a foam having less odor which was highly curable and fire retardant by use of a quaternary ammonium compound corresponding to a specified formula.
In U.S. Pat. No. 6,387,972, delayed catalytic reaction is taught to be achieved without adversely affecting the balance between blowing and gelation by using a catalyst composition that includes a specific reactive tertiary amine compound and a hydroxy-carboxylic acid salt or halocarboxylic acid salt of the specific reactive tertiary amine compound. The specific reactive tertiary amine compound must be selected from bis(dimethylaminopropyl)amino-2-propanol, bis(dimethylaminopropyl)amine, dimethylaminopropyldipropanolamine, bis(dimethylamino)-2-propanol, N,N,Nxe2x80x2-trimethyl-Nxe2x80x2-hydroxyethyl-bis(aminoethyl)ether and mixtures thereof.
In U.S. Pat. No. 6,380,274, the disclosed catalyst composition (an amide represented by a specified structure) is taught to be desirable because it has high activity, good blowing or gelling selectivity and it is bound to the urethane during the reaction so that it is not released from the final product.
In U.S. Pat. No. 5,507,480, molded polyurethane foams useful in seating applications are produced using a catalyst composition consisting essentially of 25-80 wt % pentamethyldiethylenetriamine (PMDETA) and 20-75 wt % bis(dimethylaminopropyl)methylamine. The advantages attributed to this catalyst composition include production of more open cells in the foam and the need for less gelling catalyst.
It is clear from these teachings in the prior art that selection of the optimum catalyst composition for a polyurethane-forming reaction mixture is dependent upon the particular use to which the foam product will be put and the foam characteristics necessary for that particular use.
The expression xe2x80x9cnon-moldedxe2x80x9d as used herein refers to end-use products such as water heaters in which pour-in-place polyurethane or polyisocyanurate foam is used as an insulation medium. These end-use products are foamed without the use of structural support, such as fixturing, molds or other forms of containment to prevent damage or dimensional irregularities resulting from the expansion of the reaction mixture within the cavity(ies) of the end-use product.
Many of the catalyst compositions currently used in the production of non-molded products with rigid foams generate a significant amount of pressure after the foaming reaction is complete. A contributing cause to this pressure is the ongoing generation and expansion of blowing gases after the gel point is reached. Furthermore, foam flow can be restricted when gelling occurs before gas generation/expansion is complete. This restriction in foam flow make it difficult to produce a foam having a uniform cell structure with a minimal amount of overfilling (referred to as xe2x80x9cpackingxe2x80x9d) and can damage the non-molded end-use product in which the foaming takes place.
Many of the commonly disclosed catalyst packages used in the production of rigid polyurethane polyisocyanurate foams are a combination of a blowing catalyst (i.e., a catalyst which promotes the reaction of a blowing agent such as water with the isocyanate to generate a gas such as carbon dioxide) and a gel catalyst (i.e., a catalyst which promotes the polymer-forming reaction of polyol with isocyanate). PMDETA is one of the most commonly used blowing catalysts. N,N-dimethylcyclohexylamine (DMCHA) is one of the most commonly used gel catalysts. Foams produced with a minimal level of packing made using a blowing/gelling catalyst composition such as PMDETA and DMCHA, however, are not as dimensionally stable as is desired for non-molded foam applications such as water heater insulation. Nor are the cell structure of the foams made with this catalyst combination uniform.
It would therefore be commercially advantageous to have a catalyst composition which makes it possible to produce a rigid polyurethane foam having a uniform cell structure without generating a great deal of pressure or adversely affecting the flow characteristics of the foam-forming mixture, while utilizing a minimal level of packing, particularly in the production of non-molded foams for use in end-use applications such as water heaters.
It is an object of the present invention to provide a process for the production of rigid foams in which the pressure generated during and after the foaming process is sufficiently reduced that the product foam will have a relatively uniform cell structure and the end-use product in which the foam is formed is not damaged due to excess pressure within the foam.
It is a further object of the present invention to provide a process for the production of dimensionally stable, rigid polyurethane foams in which the reaction of isocyanate with the polyol(s) is not so fast that the foam-forming mixture begins to set before the cavity of the non-molded part has been completely filled with the foam-forming mixture.
It is also an object of the present invention to provide a catalyst composition for use in the production of dimensionally stable rigid polyurethane foams.
These and other objects which will be apparent to those skilled in the art are accomplished by reacting an organic isocyanate with an isocyanate-reactive component in the presence of a blowing agent composition and a catalyst composition. The catalyst composition must include at least two blowing catalysts. Catalyst compositions in which both bis(2-dimethylaminoethyl)ether (BDMAEE) and pentamethyldiethylenetriamine (PMDETA) are present are particularly preferred. Use of an isocyanate-reactive component which includes an amine-initiated polyol and water is particularly advantageous.